Stroke is the 2nd leading cause of death throughout the world, and the leading cause of long-term disability. Ischemic stroke, the most common subtype, is caused by occlusion of an artery in the brain, resulting in the abrupt development of neurological deficits. In the first hours after stroke onset, neurological deficits can be highly unstable, with many patients improving while others deteriorate. These early changes have a major impact on long-term outcome. In fact, changes in neurologic deficits during the first 24 hours account for 40% of the variance of the 90-day modified Rankin Scale (the most widely accepted stroke outcome measure) in patients treated with tissue plasminogen activator (tPA). In my preliminary analyses, it was discovered that baseline clinical factors account for very little of the variation seen in neurologic deficits during the first 24 hours while genetic factors appear to account for over 50%. Given the finding that genetics influence the variance of early neurologic changes, it is my central hypothesis that genetics play a significant role in determining the early outcomes experienced by patients suffering from acute ischemic stroke (AIS). As a fellowship-trained emergency physician subspecializing in neurologic emergencies, my long term goal is to develop an independent research program to investigate genetic influences on acute neurological emergencies to improve patient outcomes. In order to achieve this goal, I have developed two short term goals that will be achieved during the award period: 1) to become technically proficient in genetic/genomic analysis and research methodology and 2) to develop greater familiarity with cellular and molecular mechanisms involved with brain and neurovascular injury following AIS. In order to achieve these goals, I have crafted a multidisciplinary team of mentors, advisors and collaborators. Over the past several years, I have been working closely with my primary mentor, Dr. Jin-Moo Lee (Director of the Cerebrovascular Disease and Neurointensive Care Sections in Neurology) and my secondary mentor Dr. Carlos Cruchaga (a human geneticist with expertise in complex genetic analyses), to examine possible genetic influences on the early neurologic outcomes after acute ischemic stroke. They both have a strong track record of mentoring junior investigators and bring complimentary backgrounds and expertise to my mentoring and career development. Under the guidance of Drs. Jin-Moo Lee and Carlos Cruchaga, I will engage in structured coursework, advanced training and independent study to become proficient in patient-oriented research, genetic analysis, and research methodology. To capture early neurological change following AIS, we have developed a novel quantitative endophenotype, termed ?NIHSS24h, which is a measure of the change in NIH stroke scale (NIHSS) score between baseline (<6 hours after onset) and 24 hours. Using a cohort of AIS patients treated with IV tPA that will consist of 3,000 phenotyped subjects drawn from an international multicenter consortium and representing the largest such dataset of early stroke outcomes linked to genetics in the world, I will explore the following: Aim 1: To perform genome-wide association studies (GWAS), examining early neurological change (?NIHSS24h) after AIS. Using the first 3,000 tPA-treated AIS patients (Discovery cohort, estimated accrual September, 2016), GWAS of common & rare coding variants will be performed to discover single variants, genes, and pathways associated with ?NIHSS24h. An additional 3,000 tPA-treated patients (Replication Cohort) will be accrued by September 2019, and used to replicate findings from the initial Discovery cohort. Aim 2: To use extreme phenotype sampling with low frequency functional variants of large effect size for early neurological deterioration (Aim 2a) or improvement (Aim 2b) after AIS. We will select patients with the most extremes of ?NIHSS24h, representing those with greatest deterioration or improvement, and compare to matched controls with little change in ?NIHSS24h. Replication of the top hits in each extreme will be performed in the overall cohort with gene-based sequencing and analysis. Of those patients who deteriorate, we will determine which genetic variants associate with hemorrhagic transformation and which do not. This research plan reflects my multidisciplinary training and career development in emergency medicine, neurologic emergencies, and genetics. It is a long-term objective that the unbiased approach presented in this research proposal will reveal important variants, genes and pathways involved in acute brain ischemia, possibly leading to both diagnostic and therapeutic targets to improve outcomes following stroke.